Where did it come from? 'Keep Calm and Carry On' was one of three key messages created by Britain's wartime propaganda department, the Ministry of Information, made famous as the Ministry of Truth in George Orwell's novel, 1984.

https://en.wikipedia.org/wiki/Keep_Calm_and_Carry_On

Keep Calm and Carry On was a motivational poster produced by the Government of the United Kingdom in 1939 in preparation for World War II. The poster was intended to raise the morale of the British public, threatened with widely predicted mass air attacks on major cities. Although 2.45 million copies were printed, and the Blitz did in fact take place, the poster was only rarely publicly displayed and was little known until a copy was rediscovered in 2000 at Barter Books, a bookshop in Alnwick. It has since been re-issued by a number of private companies, and has been used as the decorative theme for a range of products.

Evocative of the Victorian belief in British stoicism – the "stiff upper lip", self-discipline, fortitude, and remaining calm in adversity – the poster has become recognised around the world.